1. Field of the Invention
The present invention relates to a nitride semiconductor laser element and an external-cavity semiconductor laser device (hereinafter, referred to as an “ECL device”), and more particularly to a nitride semiconductor laser element that allows a nitride semiconductor laser element having a large output and a low reflectance of a light receiving surface to feedback light to be produced with high yields, and an ECL device using the nitride semiconductor laser element.
2. Description of the Background Art
Holographic memory technology, which is mass-capacity and very high speed data storage technology, is expected as a next generation optical memory. Researches and development toward its practical use are energetically carried out. Laser light having coherence is used for data read/write of holographic memories. Among devices using laser light, semiconductor laser elements permit significant size reduction of devices, and therefore researches and development for the use of semiconductor laser elements are in progress.
Recently, nitride semiconductor laser elements that emit blue-violet laser light having a wavelength of about 405 nm have been widely used as light sources for optical discs, such as HD-DVDs (High-Definition Digital Versatile Discs) and Blu-ray discs, having memory capacities larger than those of hitherto DVDs.
Attempts to use nitride semiconductor laser elements as light sources of holographic memories are also made. However, as different from optical discs, such as HD-DVDs and Blu-ray discs, single longitudinal mode oscillation is indispensable for a light source of a holographic memory that carries out read and write of data by making use of interference. Further, in order to perform recording and reconstructing with high repeatability when a recording medium expands and contracts because of environmental temperature and the intensity of the emission of a light source, the wavelength of a beam emitted from a light source needs to be varied (e.g., see Japanese Patent Laying-Open No. 2006-267554).
Ordinary semiconductor laser elements cannot achieve such a single longitudinal mode and wavelength varying, and therefore a DFB (Distributed Feedback) semiconductor laser element or an ECL device needs to be used. An ECL device is often used as a light source of a holographic memory because it can be relatively easily produced as compared to a DFB semiconductor laser element.
Characteristics required of an ECL device are that its output is large and its single longitudinal mode characteristic is good. To increase the output of the ECL device, a semiconductor laser element serving as a light source of the ECL device is also required to have a large output as its characteristic. For example, Japanese Patent Laying-Open No. 2005-167008 describes that an ECL device needs to have an output of 30 mW or higher, and in order to obtain this output, a semiconductor laser element needs to have an output of 45 mW or higher (see paragraph [0041] and so on of Japanese Patent Laying-Open No. 2005-167008).
To obtain a good single longitudinal mode characteristic in an ECL device, increasing the quantity of feedback light is effective in general. One of techniques for accomplishing this is to reduce the reflectance of a light receiving surface of a semiconductor laser element serving as a light source of an ECL device. Note that Japanese Patent Laying-Open No. 2005-167008 describes that by setting the reflectance of a light receiving surface of a semiconductor laser element to feedback light to the range from 0% to 10%, a single longitudinal mode laser beam can be accomplished, and in particular, it is preferred that the reflectance be 3% or less (see paragraph [0050] and so on of Japanese Patent Laying-Open No. 2005-167008).